CCITT V.32, V.32bis and V.34 are recommendations promulgated from the CCITT (Consultative Committee on International Telegraphy and Telephony) for modulation techniques that have evolved into standards for a series of higher speed modems used with personal computers. In general, these standards are based on a modified quadrature amplitude modulation technique designed to permit full-duplex transmission over a public switched telephone network (PSTN) at bit rates of 9.6 Kbps and greater. The key to operation of modems operating under these standards is a built-in echo canceling technique that enables transmitted and received signals to occupy the same bandwidth.
A problem with many PSTN echo canceling modems, however, occurs in the recognition of call termination. For example, when a user of an echo canceling modem is connected to an on-line service, the modem may have difficulty in determining when the service has disconnected. That is, to end the on-line session, upon command from the remote user, the service will disconnect. However, because of the time taken for the echo to be recognized, the remote modem will hang-on for several seconds before disconnecting itself to terminate the call. This leads to dissatisfaction on the part of the customer with respect to the modem device.
Many PSTN communications devices detect call termination by monitoring the loop current of the telephone line with devices placed in series with a loop-holding circuit. For example, current monitoring relays may be used for monitoring loop current, but such relays are generally large and relatively expensive. Modems commonly utilize a solid-state loop-holding circuit, or gyrator, in lieu of a large inductor to pull DC loop current on the telephone line from the local central office, while maintaining a high AC impedance. These solid-state devices usurp approximately 1 to 1.5 volts of tip-to-ring voltage, where this tip-to-ring voltage may only be on the order of a few volts on long subscriber lines. (At 20 mA loop current, for example, tip-to-ring voltage in the Japanese PTT tested at 6 V max, roughly 4 V for the historic 200 Ohm load in FCC Part 68 and 8 V max for 400 Ohm loads per EIA 470-A.)
The solid-state gyrators used in high-speed modems use regeneration to achieve the required high linearity for CCITT V.34 operation. As would be understood by a person skilled in the art, linearity is important at certain signal levels for maintaining spectral efficiency in terms of bits/Hz. Since the gain of bipolar and FET transistors decreases at low operating voltages, the voltage robbing solid-state series method of loop current detection is ill-suited to the linearity requirement and V.34 operation.
A digital signal processing method also exists in the prior art using an echo correlator. However, this method uses requires the uses of significant computing time and memory, and may require several seconds to determine that the far end has disconnected. Accordingly, there is a need for a quick and reliable methodology in modem devices to detect loop current interruption effectuated by the local central office on switched telephone lines.